Rapid revascularization of injured, ischemic and regenerating organs is essential to restore organ function. The angiogenic switch initiates the revascularization process and involves recruitment of endothelial cells that assemble into neovessels (Folkman et al., “Angiogenesis in Cancer, Vascular, Rheumatoid and Other Disease,” Nat. Med. 1:27-31 (1995); Folkman et al., “Therapeutic Angiogenesis in Ischemic Limbs,” Circulation 97:1108-1110 (1998); Hanahan et al., “Patterns and Emerging Mechanisms of the Angiogenic Switch During Tumorigenesis,” Cell 86:353-364 (1996); Risau, W. “Mechanisms of Angiogenesis,” Nature 386:671-674 (1997); Yancopoulos et al., “Vascular-specific Growth Factors and Blood Vessel Formation,” Nature 407:242-248 (2000); Carmeliet et al., “Angiogenesis in Cancer and Other Diseases,” Nature 407:249-257 (2000); Pepper et al., “Manipulating Angiogenesis. From Basic Science to the Bedside,” Arterioscler. Thromb. Vasc. Biol. 17:605-619 (1997). Much effort has been focused on delivering aniogenic factors to accelerate tissue revascularization (Isner et al., “Myocardial Gene Therapy,” Nature 415:234-239 (2002); Khurana et al., “Insights from Angiogenesis Trials Using Fibroblast Growth Factor for Advanced Arteriosclerotic Disease,” Trends Cardiovasc. Med. 13:116-122 (2003); Cao et al., “Angiogenic Synergism, Vascular Stability and Improvement of Hind-Limb Ischemia by a Combination of PDGF-BB and FGF-2,” Nat. Med. 9:604-613 (2003); and Carmeliet, P., “VEGF Gene Therapy: Stimulating Angiogenesis or Angioma-Genesis?” Nat. Med. 6:1102-1103 (2000). However, because tissue injury is associated with disruption of a permissive microenvironment necessary for recruiting pre-existing endothelial cells (ECs), exogenous introduction of vascular progenitors may facilitate restoration of organ revascularization. Accumulating evidence suggests that bone marrow contains vascular progenitor cells that can mobilize to ischemic sites and complement neo-angiogenesis afforded by pre-existing endothelium, thereby restoring rapid and timely organ revascularization.
Adult bone marrow is a rich reservoir of tissue-specific stem and progenitor cells. Among these, a scarce population of cells known as EPCs can be mobilized to the circulation and contribute to the neoangiogenic processes. Circulating EPCs (CEPs) have been detected in the circulation either after vascular injury or during tumor growth. CEPS primarily originate from EPCs within the bone marrow and differ from sloughed mature, circulating endothelial cells (CECs) that randomly enter the circulation as a result of blunt vascular injury. It is possible, however, that the parenchyma of the systemic vasculature or certain organs may harbor endogenous EPC-like cells. For example, distinct side population stem cells within the skeletal muscle can differentiate into ECs (Majka et al., “Distinct Progenitor Populations in Skeletal Muscle are Bone Marrow Derived and Exhibit Different Cell Fates During Vascular Regeneration,” J. Clin. Invest. 111:71-79 (2003). Therefore, CEPs may originate either from bone marrow-derived EPCs or from resident EPCs embedded within organs and the systemic vasculature. Endothelial progenitor cells residing in the bone marrow will be referred to as EPCs, while endothelial progenitors detected in the circulation will be referred to as CEPs.
Emerging evidence suggests that angiogenic factors recruit subsets of proangiogenic hematopoietic cells, including hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs). Corecruitment of HSCs and HPCs, along with EPCs and CEPs, may contribute to the initiation and sustenance of neoangiogenesis. The physiological role of corecruited HSCs and HPCs in formation of long-lasting functional neovessels remains to be determined.
Several studies have shown that bone marrow-derived cells functionally contribute to neoangiogenesis during wound healing and limb ischemia (Majka et al., “Distinct Progenitor Populations in Skeletal Muscle are Bone Marrow Derived and Exhibit Different Cell Fates During Vascular Regeneration,” J. Clin. Invest. 111:71-79 (2003); Asahara et al., “Isolation of Putative Progenitor Endothelial Cells for Angiogenesis,” Science 275:964-967 (1997); Asahara et al., “Bone Marrow Origin of Endothelial Progenitor Cells Responsible for Postnatal Vasculogenesis in Physiological and Pathological Neovascularization,” Circ. Res. 85:221-228 (1999); (Asahara et al., VEGF Contributes to Postnatal Neovascularization by Mobilizng Bone Marrow-Derived Endothelial Progenitor Cells,” EMBO J. 18:3964-3972 (1999); Iwaguro et al., “Endothelial Progenitor Cell Vascular Endothelial Growth Factor Gene Transfer for Vascular Regeneration,” Circulation 105:732-738 (2002); Kalka et al., “Transplantation of ex vivo Expanded Endothelial Progenitor Cells for Therapeutic Neovascularization,” Proc. Natl. Acad. Sci. USA 97:3422-3427 (2000); Schatteman et al., “Blood-Derived Angioblasts Accelerate Blood-Flow Restoration in Diabetic Mice,” J. Clin. Invest. 106:571-578 (2000); Crosby et al., “Endothelial Cells of Hematopoietic Origin Make a Significant Contribution to Adult Blood Vessel Formation,” Circ. Res. 87:728-730 (2000); Takahashi et al., “Ischemia- and Cytokine-Induced Mobilization of Bone Marrow-Derived Endothelial Progenitor Cells for Neovascularization,” Nat. Med. 5:434-438 (1999); Luttun et al., “Vascular Progenitors: From Biology to Treatment,” Trends Cardiovasc. Med., 12:88-96 (2002); Rafii et al., “Circulating Endothelial Precursors: Mystery, Reality, and Promise,” J. Clin. Invest. 105:17-19 (2000), postmyocardial infarction (Orlic et al., “Bone Marrow Cells Regenerate Infarcted Myocardium,” Nature 410:701-705 (2001); Orlic et al., “Mobilized Bone Marrow Cells Repair the Infarcted Heart, Improving Function and Survival,” Proc. Natl. Acad. Sci. USA 98:10344-10349 (2001); Kocher et al., “Neovascularization of Ischemic Myocardium by Human Bone-Marrow-Derived Angioblasts Prevents Cardiomyocyte Apoptosis, Reduces Remodeling and Improves Cardiac Function,” Nat. Med. 7:430-436 (2001); Jackson et al., “Regeneration of Ischemic Cardiac Muscle and Vascular Endothelium by Adult Stem Cells,” J. Clin. Invest. 107:1395-1402 (2001); Edelberg et al., “Young Adult Bone Marrow-Derived Endothelial Precursor Cells Restore Aging-Impaired Cardiac Angiogenic Function,” Circ. Res. 90:E89-E93 (2002), endothelialization of vascular grafts (Shi et al., “Evidence for Circulating Bone Marrow-Derived Endothelial Cells,” Blood 92:362-367 (1998); Bhattacharya et al., “Enhanced Endothelialization and Microvessel Formation in Polyester Grafts Seeded with CD34+Bone Marrow Cells,” Blood 95:581-585 (2000); Kaushal et al., “Functional Small-Diameter Neovessels Created Using Endothelial Progenitor Cells Expanded ex vivo,” Nat. Med. 7:1035-1040 (2001); Noishiki et al., “Autocrine Angiogenic Vascular Prosthesis with Bone Marrow Transplantation,” Nat. Med. 2:90-93 (1996), atherosclerosis (Sata et al., “Hematopoietic Stem Cells Differentiate into Vascular Cells that Participate in the Pathogenesis of Atherosclerosis,” Nat. Med. 8:403-409 (2002), retinal and lymphoid organ neovascularization (Otani et al., “Bone Marrow Derived Stem Cells Target Retinal Astrocytes and Can Promote or Inhibit Retinal Angiogenesis,” Nat. Med. 8:1004-1010 (2002); Grant et al., “Adult Hematopoietic Stem Cells Provide Functional Hemangioblast Activity During Retinal Neovascularization,” Nat. Med. 8:607-612 (2002); Crisa et al., “Human Cord Blood Progenitors Sustain Thymic T-Cell Development and a Novel Form of Angiogenesis,” Blood 94:3928-3940 (1999), vascularization during neonatal growth (Young et al., “VEGF Increases Engraftment of Bone Marrow-Derived Endothelial Progenitor Cells (EPCs) into Vasculature of Newborn Murine Recipients,” Proc. Natl. Acad. Sci. USA 99:11951-11956 (2002) and tumor growth (Asahara et al., “Bone Marrow Origin of Endothelial Progenitor Cells Responsible for Postnatal Vasculogenesis in Physiological and Pathological Neovascularization,” Circ. Res. 85:221-228 (1999); Lyden et al., “Impaired Recruitment of Bone-Marrow-Derived Endothelial and Hematopoietic Precursor Cells Blocks Tumor Angiogenesis and Growth,” Nat. Med. 7:1194-1201 (2001); Reyes et al., “Origin of Endothelial Progenitors in Human Postnatal Bone Marrow,” J. Clin. Invest. 109:337-346 (2002); Moore, M. A., “Putting the Neo into Neoangiogenesis,” J. Clin. Invest. 109:313-315 (2002); Gehling et al., “In vitro Differentiation of Endothelial Cells from AC133-Positive Progenitor Cells,” Blood 95:3106-3112 (2002); Marchetti et al., “Endothelial Cells Genetically Selected from Differentiating Mouse Embryonic Stem Cells Incorporate at Sites of Neovascularization in vivo,” J. Cell. Sci. 115:2075-2085 (2002); Davidoff et al., “Bone Marrow-Derived Cells Contribute to Tumor Neovascular and, When Modified to Express an Angiogenesis Inhibitor, Can Restrict Tumor Growth in Mice,” Clin. Cancer Res. 7:2870-2879 (2001)).
These studies have introduced the concept that vascular trauma and organ regeneration results in the release of chemokines that recruit EPCs and CEPs to the neoangiogenic site. Rapid incorporation of EPCs and CEPs accelerates vascular healing and prevents potential vascular complications secondary to thrombosis and hypoxia. Tissue ischemia results in upregulation of angiogenic factors, including vascular endothelial growth factor (VEGF)-A, which through interaction with its receptors VEGFR2 (KDR or Flk-1) and VEGFR1 (Flt-1) expressed on EPCs, CEPS, HSCs and HPCs, promotes migration of these cells to the site of the injury.
Despite the contribution of bone marrow-derived progenitors to tissue revascularization in animals models, the importance of these cells in restoring organ vascularization in a clinical setting remains unknown. Several recent clinical trials have challenged the potential of bone marrow-derived cells in restoring vascularization of ischemic tissues. The success of these strategies depends on defining the mechanisms by which stem and progenitor cells undergo appropriate molecular and differentiation, thereby permitting their functional incorporation into adult tissues.
The present invention is directed to overcoming the deficiencies in the art by providing a promoter which functions only in stem cells and not in other cell types to facilitate recovery and study of stem cells and to achieve therapeutic benefits.